Electrochemical fuel cells convert fuel and oxidant fluid streams to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly ("MEA") comprising a solid polymer electrolyte or ion-exchange membrane disposed between two porous electrically conductive electrode layers. An electrocatalyst is typically disposed at each membrane/electrode layer interface to induce the desired electrochemical reaction.
In typical fuel cells, the MEA is disposed between two electrically conductive separator plates. A fluid flow field provides a means for directing the fuel and oxidant to the respective electrocatalyst layers, specifically, at the anode on the fuel side and at the cathode on the oxidant side. A simple fluid flow field may consist of a chamber open to an adjacent porous electrode layer with a first port serving as a fluid inlet and a second port serving as a fluid outlet. The fluid flow field may be the porous electrode layer itself. More complicated fluid flow fields incorporate at least one fluid channel between the inlet and the outlet for directing the fluid stream in contact with the electrode layer or a guide barrier for controlling the flow path of the reactant through the flow field. The fluid flow field is commonly integrated with the separator plate by locating a plurality of open-faced channels on the faces of the separator plates facing the electrodes. In a single cell arrangement, separator plates are provided on each of the anode and cathode sides. The plates act as current collectors and provide structural support for the electrodes.
The fuel stream directed to the anode by the fuel flow field migrates through the porous anode and is oxidized at the anode electrocatalyst layer. The oxidant stream directed to the cathode by the oxidant flow field migrates through the porous cathode and is reduced at the cathode electrocatalyst layer.
Solid polymer fuel cells generally use fuels, such as, for example, hydrogen or methanol, which are oxidized at the anode to produce hydrogen cations. The hydrogen cations migrate through the ion-conducting electrolyte membrane and react with an oxidant such as oxygen in air at the cathode to produce water as one of the reaction products. The anode and cathode reaction equations in hydrogen/oxygen fuel cells are believed to be as follows:
Anode reaction: H.sub.2 .fwdarw.2H.sup.+ +2e.sup.- PA1 Cathode reaction: 1/2O.sub.2 +2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 O PA1 (a) an oxidant flow field associated with the cathode for directing an oxidant supply stream to the cathode between a first oxidant flow field port and a second oxidant flow field port; PA1 (b) a fuel flow field associated with the anode for directing a fuel stream to the anode between a fuel stream inlet port and a fuel stream outlet port; and PA1 (c) an oxidant stream flow switching device for periodically reversing the direction of flow of the oxidant supply stream between the first oxidant flow field port and the second oxidant flow field port without synchronously reversing said fuel stream flow direction through said fuel flow field.
Water in the ion-exchange membrane facilitates the migration of protons from the anode to the cathode. The membrane is electrically non-conductive and also serves as a barrier to separate the hydrogen-containing fuel stream from the oxygen-containing oxidant stream.
The electrons produced at the anode induce an electrical current through an external circuit or load from the anode to the cathode.
Because water is produced by the cathode reaction, as the oxidant stream travels through the oxidant flow field, the oxidant stream absorbs product water. The product water is absorbed as water vapor until the oxidant stream becomes saturated; additional product water may be carried in the oxidant stream as entrained water droplets.
The cumulative effect of product water absorption into the oxidant stream causes the flow field region near the oxidant flow field outlet to contain more water than the flow field region closer to the oxidant stream inlet. Therefore, the fresh oxidant supply stream typically enters the oxidant flow field at its driest region. If the oxidant supply stream entering the oxidant flow field is not adequately humidified, the oxidant stream may absorb water from the membrane in the region nearest the oxidant stream inlet.
It is generally well known that most conventional fuel cell ion-exchange membranes must be kept moist to maintain adequate ionic conductivity and to reduce structural damage which may result if the membrane is allowed to become too dry. It is known that leaks in membranes frequently occur near reactant stream inlet ports. Such leaks may be caused or contributed to by inlet streams drying the membrane, resulting in the formation of cracks or holes.
Accordingly, in the prior art, it is known to provide means for keeping the membrane wet and/or humidifying the reactant streams before they enter the flow fields. A disadvantage of conventional methods of humidifying the reactant streams is that incorporating an external humidification apparatus adds to the system complexity and reduces the overall system efficiency.
The production of water at the cathode may cause another problem if too much water accumulates in the oxidant flow field. If the oxidant stream becomes saturated, two phase flow may occur, that is, the oxidant stream may contain water vapor and liquid water droplets. Liquid water in the oxidant flow field can "flood" the porous electrode and obstruct the oxidant from reaching the cathode electrocatalyst. Saturation and flooding is more likely to occur in the portions of the oxidant flow field closest to the outlet, where the oxidant stream has had the most opportunity to accumulate product water.
In view of the above-identified problems, overly wet or dry regions of the flow field can detrimentally affect fuel cell performance and accelerate the degradation of performance over time. Fuel cell performance is defined as the voltage output from the cell for a given current density; higher performance is associated with higher voltage for a given current density. Accordingly, there is a problem with conventional fuel cells which have localized wet and dry regions caused by the cumulative effect of reaction product water absorption into the oxidant stream.
Two or more fuel cells can be connected together, generally in series but sometimes in parallel, to increase the overall power output of the assembly. In series arrangements, one side of a given plate serves as an anode plate for one cell and the other side of the plate can serve as the cathode plate for the adjacent cell. Such a series connected multiple fuel cell arrangement is referred to as a fuel cell stack, and is typically held together in its assembled state by tie rods and end plates.
As with single cell fuel cell assemblies, in a fuel cell stack where fixed inlets and outlets are typically used for supplying and exhausting reactants, the accumulation of product water in the oxidant flow fields causes similar localized wet and dry conditions in each individual fuel cell of the stack. Consequently, at the inlets near the stack oxidant supply manifolds, the membranes can become overly dry, while the oxidant stream can become saturated near the outlets to the stack oxidant exhaust manifolds.
Accordingly, it is an object of the present invention to provide a method and apparatus for managing product water and distributing water to an ion-exchange membrane in an electrochemical fuel cell, while reducing the above-identified problems caused by localized conditions found in conventional fuel cells and fuel cell stacks.